I. Basic Design Principles

Lightweight roof purlins (especially cold-formed thin-walled steel purlins) are sensitive to snow loads; they must be verified against the most unfavorable scenario of non-uniform snow distribution, rather than being assessed solely based on uniform distribution.

II. Classification of Common Unfavorable Snow Load Conditions

III. Load Characteristics and Design Considerations for Various Conditions

1. Single-span gable roof

Load characteristics

Snow is blown away from the windward side and accumulates on the leeward side. The snow load on the leeward side is typically 1.25 to 2.0 times the uniformly distributed snow load (depending on the slope; specific values are determined according to GB 50009).

Design considerations

When verifying purlins, calculations should not be based solely on a uniform snow load across the entire span; the unfavorable distribution scenarios for the left and right half-spans must be verified separately.

2. Multi-span gable roof (two or more spans)

Load characteristics

The concave valley where two spans meet is the area of heaviest snow accumulation. Wind-blown snow deposits in the valley, combining with snow sliding from the adjacent roof slopes to form a triangular or trapezoidal accumulation zone.

Design considerations

Purlins near the valley should be spaced more closely or have larger cross-sections;

The snow load value in this area may be taken as 2.0 to 2.5 times the basic snow pressure (depending on the span ratio and wind direction).

3. Gable roof with parapets

Load characteristics

Parapets obstruct airflow, causing snow to accumulate at the base of the parapet and form a triangular snowdrift; the peak load occurs at the base of the wall and is generally taken as 2.0 × S₀ (where S₀ is the basic snow pressure).

Design considerations

Purlins within a range of 2 to 4 meters from the parapet require reinforcement;

The length of the accumulation zone is calculated using standard formulas based on the parapet height and basic snow pressure.

4. Gable roof with skylights

Load characteristics

Skylights protruding from the roof alter the local wind field, causing snow to accumulate on the windward side of the skylight.

Design considerations

Purlins within a range of approximately 2 to 3 meters on either side of the skylight should be verified using an increased snow load;

If the skylight is tall (protruding ≥ 1 m above the roof), the accumulation effect is more pronounced.

5. Snow accumulation from sliding (sliding accumulation)

Load characteristics: Snow slides from a higher roof onto a lower roof, forming an accumulation near the high wall on the lower roof.

Design considerations

The length of the accumulation zone is generally taken as 2 × the height difference (but not exceeding 15 m);

The peak load in the accumulation zone is approximately 2.0 × S₀, with a triangular distribution (maximum at the wall base, tapering to zero at the far end).

6. Snow accumulation from drifting (wind-blown accumulation)

Load characteristics

Wind picks up snow from a higher roof or open ground and deposits it on a lower roof near the high wall. Unlike sliding accumulation, this is the result of wind-induced drifting.

Design considerations

The length of the drift zone is typically calculated as 5 to 10 times the snow drift height;

The load distribution is trapezoidal or triangular, with the peak also located at the wall base;

Sliding accumulation and snow drifting should not be superimposed; the greater of the two values is used.

Note on potential confusion

Item 5 (sliding accumulation) → Gravity-driven (snow slides down from a higher level);

Item 6 (snow drifting) → Wind-driven (wind blows snow onto the area).

The mechanisms differ; the more critical scenario is selected for design.

7. Snow accumulation around roof projections

Load characteristics

Projections (such as ventilation ducts, equipment bases, exhaust vents, etc.) obstruct snow sliding and wind clearing, causing localized snow accumulation on their windward side.

Design considerations

Purlins within a range of approximately 2–4 m on either side of the projection require reinforcement;

The accumulation height depends on the dimensions of the projection and the basic snow pressure.

Purlin design for lightweight roofs must be verified using the "most unfavorable snow distribution" rather than assuming a uniform distribution. Critical areas include the bases of parapets, junctions between high and low roofs, roof valleys, skylights, and the perimeters of projections. Each load case should be verified individually, with the most unfavorable result serving as the basis for the final design.

For more information needed or any inquiry,please feel free to contact Yumisteel team.